1. Field of the Invention
This invention relates to an electroluminescence device comprising a luminescence layer comprising (i) an organic compound having positive-hole-transporting and luminous properties and (ii) an organic compound having electron-transporting property, capable of directly converting electric energy to photo-energy under application of a voltage thereto, and providing a light emitting device having a luminescent plane surface larger than those of an incandescent lamp, a fluorescent lamp or a light emitting diode.
2. Discussion of Background
Electroluminescence devices are classified into two types depending on the exciting mechanism for the emission of light: 1) an intrinsic electroluminescence device, in which electrons and positive holes migrate locally in the luminescence layer to excite a luminescent material contained therein, capable of emitting light only in an alternating electric field, and 2) a carrier injection type electroluminescence device, in which electrons and positive holes are injected by electrodes and recombined with each other in the luminescence layer to excite a luminescent material, capable of emitting light only in a direct electric field.
In general, the intrinsic electroluminescence device contains a luminescent material made of an inorganic compound such as ZnS to which Mn or Cu is added. The electroluminescence device of this type has the drawback that a high alternating electromotive force of 100V or more is required for the operation thereof. In addition, this device is disadvantageous in that it requires high production cost, emits light with low luminous intensity, and has low durability.
On the other hand, the carrier injection type electroluminescence device has a luminescence layer made of a thin layer of an organic compound and can emit light with high luminous intensity. For instance, electroluminescence devices which emit green light are disclosed in Japanese Laid-Open Patent Application 59-194393 and U.S. Pat. No. 4,720,432, and an electroluminescence device which emits yellow light is reported in Jpn. Journal of Applied Physics, Vol. 27, Pages 713-715. These electroluminescence devices can emit light with high luminous intensity in a direct electric field even when a low electromotive force of 100V or less is applied thereto.
However, in general, an electron-transporting layer also serves as a luminescence layer in the carrier injection type electroluminescence device. Therefore, there is a limitation to the material which can be used for the luminescence layer. In addition, the durability of the conventional electroluminescence devices of this type is not sufficient yet.